Satgen231  Satellite Propagation Worldwide Part 4 by GM4IHJ    28 Aug 93
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FIELD ALIGNED propagation irregularities are often encountered as Sporadic
E ( mentioned in Part 3 ) fades, or , at end of an Aurora (Early evening
part 1 or early morning part 2 auroral sequence ). As the associated dense
patch of E layer ionisation begins to disperse, it can migrate between the
magnetic field lines but not through them. The ionisation can therefore
form a near vertical plate relative to the Earth in Northern latitudes or
it can form a horizontal plate or duct ( flux tube is the term used in
some descriptions ) over Equatorial regions.
At high latitudes the FAI curtain is not of any great importance to
satellite users as its life time is brief usually no more than ten
minutes. But it can be used by terrestrial station on the Equatorial low
latitude side of the FAI to scatter propagate to station whose normal ray
to the FAI is reciprocal to that from the other station . By carefully
point at the FAI ( not at one another ) two terrestrial stations can get
useful albeit brief contacts. Latitudes 30 to 45 are favoured for this
type of DX . Note GM4IHJ has detailed information on European and North
American FAI contacts but nothing from the Southern Hemisphere.
At lower latitudes the situation is quite different.The magnetic field
lines are horizontal above the Equator . A station in say Puerto Rico can
aim his satellite antenna at the north end of a flux tube and read the
signals direct of an Argentine station try to access Oscar on 2m ,
unknowingly accessing the other end of the flux tube. Please note it is
the other chaps UPLINK you hear. This type of propagation seems to be
allied to the Trans Equatorial successes of Greek and South African
stations. Some observers refer to Equatorial FAI propagation as
"Equatorial Aurora" because the signal has a rough tone. The use of the
word Aurora may be incorrect. Radio Amateurs are too fond of reporting
Auroral tone signals when what they really have is rough T1 or T2 signal
tone but no evidence what so ever of Aurora.
FARADAY EFFECT One major problem for satellite users which affects all
signal transitting the ionosphere , up or down , is Faraday effect
Faraday effect occurs when the plane of polarisation of the signal is
rotated naturally as it transits the ionised plasma. So what leaves the
satellite as vertical polarisation will make many rotation if HF before it
reaches you, and it probably will not be vertical for you. ( Be careful
not to mix this up with satellite attitude - Sat vertical when on your
horizon is not your ground vertical , for RS12 it is 30 degs off). At
higher frequencies the number of rotations of polarisation is reduced . It
can be 4 or 5 for a 2m signal coming at low elevation through the
ionosphere , but is somewhat less as it comes down from a satellite going
nearly overhead. This change as the satellite comes towards or away from
you has the nasty side effect that the polarisation from a low earth
satellite is always changing . Geosats change much more slowly because the
ionosphere under them is what is moving across the line of sight , not the
geosat. UHF sats have less than a full rotation of polarisation on a low
pass and microwave sats have almost none at all, but please remember that
even with microwave low earth orbit sats, the relationship between your
station vertical and the satellite vertical are slowly changing. Also note
that geosats like Astra over the Equator must set their antennas in
respect of UK vertical , not Equatorial vertical. 73 de GM4IHJ @ GB7SAN